UN Secretary-General Antonio Guterres has warned the world is facing "a direct existential threat" and must take action in the next two years to avert the disastrous consequences of runaway climate change.

Speaking in New York on Monday, Guterres said climate change was "moving faster than we are" and decried the lack of global leadership to address the issue.

"If we do not change course by 2020, we risk missing the point where we can avoid runaway climate change, with disastrous consequences for people and all the natural systems that sustain us," he said.

People everywhere are experiencing record-breaking temperatures, he continued, and extreme heatwaves, wildfires, storms and floods "are leaving a trail of death and devastation".

Guterres said scientists have been warning about global warming for decades, but "far too many leaders have refused to listen - far too few have acted with the vision the science demands".

'Greatest challenge of our time'

World leaders who signed the Paris agreement on climate change in 2015 committed to a series of measures to limit global temperature rises to less than 2 degrees Celsius and to below 1.5 degrees by the end of the century.

But recent studies show the world is off track and likely to miss that target.

Guterres said that when he addresses world leaders at the annual UN General Assembly gathering in two weeks, he will tell them "that climate change is the great challenge of our time" and what is missing is leadership and a sense of urgency to respond.

US President Donald Trump dealt a setback to the UN push for climate action when he announced last year that his country was withdrawing from the Paris Agreement.

But UN officials fear backsliding from other countries such as Australia, one of the world's worst per capita greenhouse gas polluters, which has scrapped plans to enshrine targets for reducing carbon emissions into law.

Poland, which will host the COP 24 climate summit in December, is struggling to break free of coal as its main source of energy.

A recent UN study said commitments under the Paris Agreement represent just a third of what is needed to meet the target of a cooler planet, said Guterres.

"The mountain in front of us is very high," he said. "But it is not insurmountable."

The UN chief described the upcoming COP 24 summit in the Polish city of Katowice as a "key moment" when leaders will be asked to "show they care about the people whose fate they hold in their hands".